1. Field Of The Invention
The present invention relates generally to the field of scintillation cameras and more particularly to a method and apparatus for the dynamic modification of the spatial distortion correction capabilities of a scintillation camera having spatial distortion corrections apparatus.
2. Description Of The Prior Art
The correction of scintillation cameras and the like for the inherent spatial distortion characteristics of the apparatus is important to avoid localized image event compression or expansion in the image data that appears as erroneous variations in image intensity. Spatial distortions are the result of systematic errors in the positioning of scintillation events largely due to inaccuracies and non-linear output data in the image event position coordinate data of the scintillation camera detection apparatus. The spatial distortion characteristics vary as a function of the position of the occurrence of the image event on the camera face and thus the image events are not recorded in their correct location with respect to the overall image. Even though the displacement of individual events are not visually apparent in the image, spatial distortion causes noticable image field non-uniformities apparent as intensity variations in the image.
Various spatial distortion (linearity) correction apparatus and methods have been proposed to modify image event position coordinate data of scintillation cameras to provide corrected image event coordinate data of scintillation cameras. The proposed methods provide corrected image event coordinate position data and thus correct for the inherent spatial distortion characteristic of scintillation cameras including those known as the "Anger-type".
For example, spatial distortion correction methods and apparatus have been proposed as discussed and described in U.S. Pat. No. 3,745,345 which issued to G. Muehllenher on July 10, 1973; U.S. application Ser. No. 051,176 filed by E. W. Stoub et al on June 22, 1979; "Removal Of Gamma Camera Non-Linearity And Non-uniformities through Real-Time Signal Processing" by G. F. Knoll et al, presented at the July, 1979 Paris Conference on Nuclear Medicine; "On-Line Digital Methods For Correction Of Spatial And Energy Dependent Distortion Of Anger Camera Images" Shabason et al, A Review Of Information Processing In Medical Imaging, Fifth International Conference pp. 376-388, Vanderbilt University, Nashville, Tenn., June 27-July 1, 1977; and "Quantitative Studies With The Gamma Camera; Correction For Spatial And Energy Distortion," Soussaline et al, A Review Of Information Processing In Medical Imaging, Fifth International Conference, pp. 360-375, Vanderbilt University, Nashville, Tenn., June 27-July 1, 1977.
While the hereinbefore mentioned proposed methods and apparatus of the prior art are generally suitable for their intended use and describe arrangements to provide spatial distortion correction, these arrangements do not provide compensation or modification for the changes in the spatial distortion characteristics of the scintillation camera apparatus for image events having energy levels different from the energy level at which the spatial correction factors are calculated.
Thus, the prior art arrangements are neither optimized for nor are capable of compensating for spatial distortion characteristics that vary as a function of the image event energy level; whether the difference in energy levels are due to a source having a different energy level than that for which the correction factors were calculated or in the case of a source having multiple energy levels. Accordingly, the correction factors in the proposed prior art apparatus and methods require recalculation and storage for each energy level source with which the scintillation camera is to be utilized.
Other proposed prior art studies, methods and arrangements are directed to the problem of non-uniformity of scintillation camera response resulting in the energy response of the camera varying as a function of the image event position. These prior art studies, methods and arrangements are concerned with non-uniformity correction by the use of sliding energy window techniques, spatially variant adaptive energy discrimination, and image event energy signal modification.
For example, U.S. application Ser. No. 096,181 filed by R. Arseneau on Nov. 20, 1979 is directed to a method and apparatus for the correction of non-uniformities in energy response of scintillation cameras, the non-uniformity being the variation of energy response as a function of the image event position. The energy correction method determines energy correction factors in an off-line test, measurement and analysis phase and stores the determined energy correction factors in a memory of on-line energy correction apparatus. The on-line energy correction apparatus modifies the energy signal of each image event in accordance with the energy correction factor corresponding to the position of the image event. The corrected energy signals are then processed by an energy window analyzer of fixed width to decide whether to accept or reject a particular image event.
Another method for correcting for non-uniformity is described in "A New Method Of Correcting For Detector Non-Uniformity In Gamma Camera," Lapidus, Raytheon Medical Electronics, ST-3405, November, 1977, presented at a meeting of the Southeastern Chapter of the Society of Nuclear Medicine on Oct. 15, 1977. This method modifies the Z energy signal of an image event by reading out a stored correction factor corresponding to the image event position. The correction factors vary the width of the Z signal via a pulse width modulator to supply a variable pulse width Z signal. The display apparatus utilizes the variable pulse width Z signal to vary the intensity of the displayed image point on film.
However, the aforementioned spatial distortion corrections and non-uniformity correction systems do not provide modification of spatial distortion correction factors on the basis of the energy levels of the image event signals.